WiMAX’s full name is World Interoperability for Microwave Access, another name for WiMAX is 802.16.
IEEE802.16 standard, also known as WiMAX, or Broadband Wireless Access (BWA) standard. It is a wireless metropolitan area network (WMAN) technology, which is a new air interface standard for microwave and millimeter-wave frequency bands. It is used to connect 802.11a wireless access hotspots to the Internet, and can also connect environments such as companies and homes to wired backbone lines. It can be used as a wireless extension technology for cable and DSL to achieve wireless broadband access.
WiMAX Technology Brief
WiMAX is a wireless metropolitan area network access technology based on the IEEE 802.16 standard. Its signal transmission radius can reach 50 kilometers, basically covering the suburbs.
It is precisely because of this long-distance transmission feature that WiMAX will not only be a technology for wireless access, but also a wireless extension of wired network access (Cable, DSL), and easily realize network connections in remote areas. Due to the low cost, combining this technology with microwave equipment that requires authorization or exemption will expand the broadband wireless market and improve the awareness of enterprises and service providers.
The WiMAX Forum was initiated by many wireless communication equipment/device suppliers in 2001. It is a non-profit organization headed by Intel. The goal is to promote the application and promotion of broadband wireless networks specified by the IEEE802.16 standard and raise public awareness of the potential of broadband.
To ensure the interoperability between broadband wireless access equipment from different manufacturers that adopt the same standard, and urge suppliers to solve equipment compatibility issues, thereby accelerating the utilization rate of WiMAX technology, and making WiMAX technology the industry’s best use of IEEE802.16 series broadband wireless equipment standard.
WiMAX advantages
The advantage of WiMAX enables a longer transmission distance. The 50 kilometers wireless signal transmission distance that WiMAX can achieve is unmatched by wireless local area networks.
The network coverage area is 10 times that of 3G transmission towers. As long as a few base stations are built, the entire city can be covered, which greatly expands the range of wireless network applications.
For higher-speed broadband access, the maximum access speed that WiMAX can provide is 70M, which is 30 times the broadband speed that 3G can provide.
Excellent last-mile network access service, as a wireless metropolitan area network technology, it can connect Wi-Fi hotspots to the Internet, and can also be used as a wireless extension of wired access methods such as DSL to achieve last-mile broadband access Into.
WiMAX can provide services within a linear area of 50 kilometers, and users can establish broadband connections with base stations without cables.
Multimedia communication services, because WiMAX has better scalability and security than Wi-Fi, and can realize carrier-class multimedia communication services.
Based on the above advantages, WiMAX will be able to provide users with true wireless broadband network services, even mobile communication services.
WiMAX's network architecture
The goal of WiMAX’s network architecture is to build an all-IP-based WiMAX end-to-end network architecture based on IEEE-802.16 and IETF protocols, including reference models, reference points, and modular functional decomposition to meet operationally fixed/nomadic /Portable/Simple Mobile/Full Mobile Mode requires a variety of broadband application scenarios, meets the needs of various existing services with different levels of QoS, and interconnects with existing wired or wireless networks.
WiMAX physical form
The network entities are as follows.
Access network ASN: As a logical entity, it manages the IEEE802.16 air interface and provides wireless access for WiMAX users.
Connection service network CSN: CSN is a combination of network functions, and WiMAX users provide IP connections. It is composed of the router, AAA proxy or server, user database, Internet gateway equipment, etc.
As a new network entity of the brand-new WiMAX system, it can also use some existing network equipment to realize the CSN function.
Network access provider NAP and network service provider NSP: Support multiple NSPs to share one or more ASN networks in the same NAP, and support one NSP to connect with multiple ASNs managed by multiple NAPs. Application of WiMAX The core value of the WiMAX technology system lies in metropolitan area coverage and large-bandwidth transmission. WiMAX is mainly suitable for areas and cities that are not covered by wired networks to solve the problem of broadband access for the fixed population and broadband access for the mobile population. , Especially suitable for wireless high-speed access to metropolitan area networks.
WiMAX is applicable to two types of occasions: when there is no other cable coverage method such as DSL, and when the transmission bandwidth of the existing DSL and mobile network is insufficient.
There are three main types of business.
Fixed wireless access
As a supplement to wired access methods such as DSL, access to WiMAX networks at fixed locations.
Seamless wireless access
It is suitable for people with high mobility such as business people, and industries with strong mobile office requirements such as transportation and logistics. WiMAX networks can be accessed at different locations.
Roaming mobile access
With the development of WiMAX, 802.16E will have better mobile network access features; it can realize seamless roaming on foot or in vehicles.
Prospects of WiMAX To obtain better applications in the future, WiMAX must solve the problem of frequency allocation, the cost-effectiveness of the system and the terminal, and the killer application.
At the same time, the frequencies used by WiMAX are different in different countries and are open public frequency bands in some countries. The effective allocation and management of WiMAX frequency will become an important factor affecting the development of WiMAX.
Reference model
The air interface defined by the IEEE 802.16 standard consists of a physical layer and a MAC layer. The MAC layer is independent of the physical layer and can support a variety of different physical layer specifications to adapt to various application environments.
The physical layer is composed of a transmission convergence sublayer (TCL) and a physical medium-dependent sublayer (PMD). Generally speaking, the physical layer mainly refers to PMD. TCL segments the received MAC layer data and encapsulates it into a TCL protocol data unit (PDU). PMD specifically performs a series of processing procedures such as channel coding, modulation, and demodulation. The MAC layer adopts a hierarchical structure and is divided into three sublayers, the specific service convergence sublayer (CS), the common part sublayer (CPS), and the security sublayer.
1) The CS sub-layer is responsible for interfacing with the upper layer and converging different services of the upper layer. It converts and maps the external network data received through the service access point (SAP) into a MAC service data unit, and transmits it to the SAP of the MAC layer. The protocol provides multiple CS specifications as interfaces with various external protocols, which can realize the transparent transmission of protocol data such as ATM and IP.
2) The CPS sublayer implements the main MAC functions, including system access, bandwidth allocation, connection establishment, and connection maintenance. It receives data from various CS layers through the MAC layer SAP and classifies them into specific MAC connections while implementing QoS control on the data transmitted and scheduled on the physical layer.
3) The main function of the security sublayer is to provide authentication, key exchange, encryption, and decryption processing. This sub-layer supports 128-bit, 192-bit, and 256-bit encryption systems, and uses digital certificate authentication to ensure the safe transmission of information.
WiMAX technical characteristics
(1) Wide application frequency
802.16 technology can be applied to a very wide frequency band, including 10-66GHz frequency band, <11GHz frequency band, and <11GHz unlicensed frequency band.
(2) Flexible modulation method
In the 802.16 standards, three physical layer implementation methods are defined: single carrier, OFDM, and OFDMA.
(3) Perfect QoS mechanism
In the 802.16 standards, a relatively complete QoS mechanism is defined in the MAC layer. The MAC layer can set different QoS parameters for each connection, including indicators such as rate and delay.
WiMAX technology
OFDM and OFDMA
The basic idea of OFDM technology is to divide the available bandwidth of the channel into several orthogonal sub-carriers and perform data transmission on each sub-carrier independently, thereby realizing low-speed parallel transmission of high-speed serial data streams.
It evolved from the traditional frequency division multiplexing (FDM) technology. The difference is that OFDM uses DFT (Discrete Fourier Transform) and IDFT instead of traditional band-pass filters to achieve the division between sub-carriers.
The sub-carriers can partially overlap but still maintain orthogonality, thus greatly improving the spectrum utilization of the system.
In addition, the low-speed parallel transmission of data enhances the ability of OFDM to resist multipath interference and frequency selective fading.
On the basis of OFDM technology, combined with frequency division multiple access (FDMA), the available sub-carrier resources in the channel bandwidth are allocated to different users, which is OFDMA.
Multi-antenna technology
Multi-antenna technology can double the channel capacity without increasing the system bandwidth, thereby achieving a higher data transmission rate and larger coverage, or improving signal transmission quality. The multi-antenna technology supported by the 802.16 standard includes two categories, multiple-input multiple-output, and adaptive antenna systems.
Adaptive modulation and coding
The time-varying and fading characteristics of the wireless channel determine that the channel capacity is a time-varying random variable. To make the best use of the channel capacity, the transmission rate must be changed accordingly, that is, the coding and modulation method should have adaptive characteristics.
Adaptive Modulation and Coding (AMC) technology dynamically adjusts coding and modulation methods according to channel conditions to improve transmission rate or system throughput.
The basic method is to use high-order modulation and high coding rate (such as 64QAM, 5/6 code rate) based on the measurement results of the channel quality when the channel conditions are good to achieve a higher peak rate; and when the channel conditions are poor When using low-order modulation and low coding rate (such as QPSK, 1/2 code rate) to ensure transmission performance.
Improving performance by changing the modulation and coding method instead of the transmission power can also reduce the additional interference introduced by the increase in the transmission power to a large extent.
Hybrid automatic repeat request
Hybrid automatic repeat request (H-ARQ) is a technology that combines automatic repeat request (ARQ) and forward error correction coding. It can be used to reduce the negative impact of channel and interference jitter on data transmission.
The basic working process of H-ARQ is as follows
I-Connect one or more MAC layer data units to be sent in series, and encode according to the specific specifications of the physical layer to generate 4 H-ARQ sub-packets.
The J-base station only sends one sub-packet at a time. Since there is a great correlation between the 4 sub-packets, the receiving end can decode correctly without obtaining all the sub-packets.
Therefore, after receiving the first sub-packet, the terminal tries to decode it. If the decoding is successful, the terminal immediately sends back an acknowledgment (ACK) message to the base station to prevent it from sending subsequent sub-packets.
If the decoding fails, the terminal sends back a negative (NACK) message, requesting the base station to send the next sub-packet, and so on. The terminal will decode all sub-packets received each time to improve the decoding success rate.
It can be seen that H-ARQ uses the simplest stop-and-wait retransmission mechanism to reduce control overhead and transmit and receive buffer space. If you use the OFDMA physical layer at this time, you can cleverly overcome the shortcomings of the stop-and-wait protocol channel utilization. Therefore, the agreement only stipulates that the OFDMA physical layer provides support for H-ARQ.
Power Control
802.16e stipulates that power control must be carried out in both the uplink and the downlink to comprehensively improve the performance of the system. The total transmission power consists of a fixed part and a dynamic adjustment part.
Media access mechanism
The design of the media access control mechanism is a question that must be considered for any wireless access system that adopts the shared channel mode. Different from the carrier sense/collision avoidance (CSMA/CA) strategy of IEEE 802.11, the method adopted by IEEE 802.16 is to fragment time resources at the physical layer and distinguish between uplink and downlink by time.
Each physical frame has a fixed frame length and consists of two parts: uplink and downlink. The switching point between uplink and downlink can be adjusted adaptively through the control of the MAC layer.
In TDD mode, each frame consists of n time slots. The downlink is broadcast, and the uplink is sent from the SS to the BS. Downstream comes first and upstream comes second. For broadband wireless access systems, this media access mechanism takes into account both flexibility and fairness. Each SS has the opportunity to send data, avoiding the phenomenon of a long-term lack of channel competition;
Secondly, each SS only sends data within its own sending period, which can ensure that only one data stream is transmitted on the media at any time; again, this mechanism facilitates the control of QoS, service priority, and bandwidth.
QoS guarantee mechanism
WiMAX is the first wireless access standard proposed to provide a QoS guarantee at the MAC layer. As we all know, the influence of factors such as multipath and fading on the wireless channel will lead to a higher bit error rate and packet loss rate, and the reliability and effectiveness of data transmission are difficult to guarantee.
In order to meet the higher requirements of high-speed multimedia services for indicators such as delay, bandwidth, and loss rate, WiMAX’s MAC layer defines a series of strict QoS control mechanisms, which can provide different quality services for different services in the wireless access network. At the same time, this service is connection-oriented.
Switch
When the mobile subscriber station (MS) moves away from the coverage area of the original BS or other BSs can provide better quality services, a handover (HO) procedure needs to be performed. Through the network topology message broadcast by the BS, the MS can obtain the DCD/UCD information of the neighboring cells.
The BS can also allocate a scanning period for the MS to scan and range neighboring base stations, evaluate the quality of its physical layer channels, and determine potential target BSs for possible handovers. The actual handover process can be initiated by the MS or by the BS, and the handover is a hard handover.
In addition, IEEE 802.16e defines two optional handover modes, macro diversity handover (MDHO) and fast BS handover (FBSS).
MDHO allows the MS to communicate with multiple BSs at the same time to obtain diversity gain and improve link quality. In FBSS, the MS does not need to perform a regular handover process and can realize fast handover between any two BSs in a BS set.
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